Years ago, telecommunications companies provided service to their subscribers strictly by copper wire. Within the recent decades, however, telecommunications companies have been gradually replacing much of the copper wire with optical fiber. Optical fiber permits a greater capacity of signals to travel further with considerably less degradation than when using copper wires.
The block diagram of FIG. 1 shows generally a communication system that does not include fiber. A central office (CO) 110 provides, through local digital switch (LDS) 112, subscriber service on communication path 114 to a remote terminal (RT) 118. A number N of RTs 118.sub.n (n=1 . . . N) can be coupled to the switch 112 via a respective communication path 114.sub.n. Each communication path 114.sub.n includes T1 lines, i.e., lines capable of carrying signals according to the DS1 signaling standard for transmission at 1.544 Mbps. A T1 facility can support 24 simultaneous DS0 channels, where DS0 is a standard for transmission (64 Kbps) for PCM digitized voice channels and is well known in the art.
The RTs each respectively contain a number of different cards including "plain old telephone service" ("POTS") cards 122, which are each in turn coupled to a respective subscriber's home or office to provide telephone or other communication service. The connection 124 between a respective POTS card 122 and the subscribers location is often referred to herein as a "drop." Each drop is composed of a "tip" line and "ring" line. POTS cards can often support more than one drop.
Often, the telecommunication service provider (e.g., a telephone company) will need to test an individual "drop" from the RT 118.sub.n to the individual subscriber's location. Rather than having to go to each subscriber's location, equipment is provided at the CO to enable remote testing of drops, including a "mechanized loop test" (MLT) unit 130. The MLT 130 has a number of DC test pairs 132, formed of copper, coupled between the MLT 130 and switch 112. While two test pairs 132 are shown in FIG. 1, one or more test pairs are often provided. In addition, dedicated test lines, referred to herein as "bypass pairs" and also formed of copper, are coupled between the CO 110 and the RTs such that each RT 118.sub.n receives its own respective bypass pair 134.sub.n. The switch 112 switches to couple a DC test pair to a bypass pair such that only a single RT unit is coupled to the respective DC test pair at a given time.
Tests are performed under control of the MLT 130. Generally, to initiate a test, first switch 112 directs that 130 volts be placed on the tip line of the individual drop to be tested via its respective POTS card. This 130 volts informs the selected POTS card in the RT that its drop is about to be tested. The POTS card then redirects its connection from the communication path 114.sub.n to the bypass pair coupled to the RT. Then the MLT 130, having been electrically coupled to the drop to be tested via the appropriate bypass pair 134.sub.n and switch 112, takes appropriate electrical measurements over the drop under test (e.g., by placing a voltage or current on the bypass pair 134.sub.n).
The telecommunications industry has gradually been replacing many of their copper wire connections with optical fiber, and particularly those connections between the CO and the RTs. Referring to the block diagram of FIG. 2, central office 210 is coupled to each of N RT units 218.sub.n, n=1 . . . N, via a communication path 216.sub.n formed of optical fiber. (In one implementation currently provided by DSC Communications Corporation, N.ltoreq.5). The communication path 216.sub.n carries signals according to the SONET standard of optical network transmission as is known in the art. In the CO 210, a local digital switch 212 and MLT 230 are still present and coupled to one another, the MLT 230 providing DC test pairs 232 to the LDS 212. However, rather than being directly connected to each RT via copper lines, the switch 212 is coupled with copper T1 lines 214, that are capable of carrying signals in accordance with DS1 or DS0, to a central office terminal (COT) 240. The COT is then coupled to each RT via fiber communication paths 216.sub.n. The COT 240 also receives one or more bypass pairs 234, formed of copper wire, from LDS 212.
Despite the use of fiber paths 216.sub.n, MLT 230 as used by most telecommunication service providers is the same MLT used when a copper wire connection was formed between the central office and each RT unit. Since the MLT 230 cannot take measurements over fiber (it can only take electrical measurements), testing the individual drops becomes difficult when fiber is installed. Thus, equipment has been developed to mimic copper signals over the fiber path, enabling switch 212 to essentially "perceive" a copper bypass pair from the central office to each RT and to allow the POTS cards at each RT to essentially "perceive" the switch 212 as if coupled with copper wire. This equipment includes COT 240, mentioned above.
The COT 240 includes a common control unit 242 as well as one or more card banks 244.sub.m, m =1 . . . M. In one implementation currently provided by DSC Communications Corporation, M.ltoreq.9. Common control unit 242 provides hardware, firmware, and/or software needed to interface the copper lines 214 and bypass pairs 234 from the LDS 212 to optical fiber paths 216.sub.n. Each card bank within the COT 240 can also be one of a variety of types, e.g., a channel bank, a fiber bank, or the like. In FIG. 2, each of the card banks 244.sub.m in the COT is shown as a channel bank. Card banks 244.sub.m, each include slots for housing various line cards. In one implementation, each channel bank includes 56 line card slots. In the case of a channel bank, e.g., 244.sub.M, line cards may include POTS cards 246 coupled to a drop 248.
Each RT unit 218.sub.n also includes a common control unit 260, which is similar in many respects to common control unit 242 in COT 240. Each RT 218.sub.n also includes a plurality of card banks 262.sub.1k, 262.sub.Np (k=1 . . . K, p=1 . . . P), where the subscript for each card bank 262 identifies first the RT number and then the bank number (262.sub.(RT#)(bank#)). The number (K, P) of card banks 262.sub.1k, 262.sub.Np in each RT 218.sub.n can vary, although in one implementation, K, P.ltoreq.9. Each card bank within each RT can also be one of a variety of types: the card banks can be either channel banks, fiber banks, or the like. For example, in FIG. 2, RT-1218.sub.1 is shown to contain one card bank 262.sub.11, which is a channel bank. Channel bank 262.sub.11 includes a number of line cards, including POTS cards 247 which are each coupled to a drop 224. RT-N 218.sub.N, however, includes P card banks 262.sub.Np, where at least one of the banks 262.sub.Np is a channel bank, housing POTS cards 247, and at least one of the banks is a fiber bank 262.sub.N1.
A fiber bank, e.g., 262.sub.N1, includes a number of fiber cards (not shown), which convert electrical signals to optical signals and vice versa. Each card in the fiber bank 262.sub.N1 is coupled to an optical network unit (ONU) 270.sub.q, q=1. Q, via a fiber connection. As they receive optical signals, ONUs 270.sub.q are generally used to provide telecommunication services to subscribers that are located too far away from the RT to receive reliable service over copper lines. Each ONU 270.sub.q includes a fiber card (for converting optical signals into electrical signals and vice versa, not shown) and a number of POTS cards 247 each coupled to a respective drop 224. Each fiber bank, e.g., 262.sub.N1 can have a plurality of ONUs coupled to it, and in one implementation 16 ONUs can be coupled to a fiber bank such as 262.sub.N1. Thus, in a system where nine card banks can be included in an RT, and if all card banks are fiber banks, then up to 144 ONUs can be coupled to a single RT.
In order to mimic the bypass pairs for testing purposes, line cards 250, referred to as bypass cards (BYPP cards), are supplied. The bypass cards are deployed in pairs: one is provided in a channel bank 244.sub.m at the COT 240 and a companion card is provided at a channel bank of an RT 218.sub.n. The bypass cards at the COT translate MLT signals, received via a bypass pair 234, into optical signals (signals that can be sent over fiber) and translate optical signals received from the communication path 216.sub.n into electrical signals that can be used by the switch 212. Similarly, bypass cards at the RT translate optical signals received from communication path 216.sub.n into electrical signals that can be used by the drops 224 and vice versa. Once pairs of bypass cards are deployed, a connection between the pair is then permanently "nailed up" across the SONET connection. In other words, when deployed, a connection between the pair of BYPP cards is established and will permanently exist. A "nailed up" connection between a pair of BYPP cards is represented in FIG. 2 with dashed line 272. Each "nailed up" connection between BYPP cards 250 is composed of a pair of DS0 channels.
Although not shown in FIG. 2, each bypass card 250 in the COT is coupled to a bypass pair 234. Each bypass pair 234 may be coupled to more than one bypass card 250.
In order to test all the drops 224 from the RTs 218.sub.n, each set of channel banks in each RT 218.sub.n receives at least one BYPP card 250. Since the BYPP cards 250 are deployed in pairs, for every BYPP card in an RT, there is a corresponding BYPP card at the COT 240. Thus, if five RTs are coupled to the COT, and each RT includes nine channel banks each respectively containing a BYPP card, then 45 BYPP cards will be required to be installed in the COT channel banks. Since each channel bank at the COT has only a limited number of slots, e.g., 56, additional channel banks will usually be necessary to support line cards for subscriber services. Moreover, each pair of BYPP cards (one card located at the COT and a second card located at an RT) has a "nailed up" connection between them. Thus, BYPP cards will take up slots and bandwidth that could otherwise be used for subscriber service. Yet, despite the use of considerable COT and bandwidth resources, testing is infrequent.
The problem becomes magnified with the use of ONUs 270.sub.q, While only one BYPP card 250 is needed per set of channel banks in each RT unit, BYPP cards cannot be installed in fiber banks to support testing of ONUs connected to that fiber bank. Rather, each ONU receives its own BYPP card. Since a relatively large number, e.g., 144, ONUs can be supported by any one node, companion cards at the COT 240 for each ONU 270.sub.q and permanent "nailed up" connections between them will also have to be provided. Thus, considerable space will be dedicated to BYPP cards 250 at the COT, which can only accommodate a limited number of channel banks (e.g., 9). For instance, if all 144 ONUs are supported off of an RT, approximately three channel banks (where each contains 56 slots), or 1/3 of the COT resources, will be dedicated to test functions that are infrequently performed.
Therefore, existing alternatives to copper bypass pairs are becoming prohibitively expensive and wasteful of system resources (e.g., card slots and bandwidth). The ability to test drops according to these alternatives requires establishing and maintaining distinct connections between each COT BYPP card and a respective RT BYPP card, whether or not a test is in progress.